In situ visualization of the performance of a supercritical-water salt separator using neutron radiography

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J. of Supercritical Fluids 43 (2008) 490–499

In situ visualization of the performance of a supercritical-water salt separator using neutron radiography Andrew A. Peterson a,b , Peter Vontobel b , Fr´ed´eric Vogel b,∗ , Jefferson W. Tester a a

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA b Paul Scherrer Institut, Villigen, Switzerland Received 5 March 2007; received in revised form 1 August 2007; accepted 2 August 2007

Abstract Salt separation from supercritical water has been identified as a key issue in the deployment of supercritical-water technologies, particularly in the supercritical-water gasification of biomass. In order to better understand salt separation, neutron radiography has been employed to allow visualization of the transport phenomena associated with the separation of salt from supercritical D2 O in a reverse-flow vessel. D2 O was used as a surrogate for H2 O because of its low neutron-attenuation coefficient and similar physical properties. Salts that formed a brine as a second phase, such as Na2 B4 O7 and K3 PO4 , performed well in the separator. Na2 SO4 , which forms a solid precipitate, created more issues and caused blockage of the separator under certain conditions. © 2007 Elsevier B.V. All rights reserved. Keywords: Supercritical water; Salt; Neutron radiography; Precipitation

1. Introduction A process has been developed at the Paul Scherrer Institut (PSI) which produces renewable methane from biomass by catalytic gasification in supercritical water [1,2]. The researchers have demonstrated that woody biomass can be gasified into methane, hydrogen, and carbon dioxide with 99% of the carbon in the biomass converted into product gases. This process, known as supercritical-water gasification (SCWG), uses a catalyst, such as skeletal nickel or ruthenium supported on carbon, to achieve these high efficiencies, and operates at temperatures of around 400 ◦ C and 30 MPa pressure. In supercritical water the solubilities of most ionic species become very low and as a result many salts that are present in the feedstock will tend to precipitate from the solution. These salts may precipitate on the catalyst, which reduces its effective surface area and therefore its activity. In order to adapt this supercritical-water biomass gasification process to accept a wider range of feedstocks that may contain salts, such as manure, a means of managing (including separating and removing) the salts must be devised.

∗

Corresponding author. Tel.: +41 56 310 2135; fax: +41 56 310 2199. E-mail address: [email protected] (F. Vogel).

0896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2007.08.003

In this study, we use neutron radiography to analyze the transport and separation behavior of salt–water mixtures inside of a supercritical-water/salt separator. 1.1. Salt behavior in supercritical water The properties of water change as it is heated to and above its critical point. In particular, the dielectric constant () and ion dissociation constant (KW ) drop drastically, as seen in Fig. 1. This changes the solvating properties of water, resulting in increased solubility of organic compounds but decreased solubility of ionic compounds [3]. Therefore, most salts are only scarcely soluble in supercritical water. In the catalytic supercritical-water gasification of biomass, salts can precipitate on the surface or in the the pores of the catalyst, drastically decreasing the catalyst activity. In tests with distillers’ dried grains and solubles as a feedstock, Elliott et al. [4] found evidence of deactivation of their ruthenium catalyst after several hours online. Upon inspection of their catalyst with X-ray photoelectron spectroscopy, accumulation of sulfates was found. Elemental mapping using scanning electron microscopy and energy-dispersive X-ray microanalysis showed the sulfur deposits to be highly associated with the ruthenium on their catalyst. Additionally, a “crust” of Mg and P was found to surround their catalyst pellets. Salt management issues are inherently a part of supercritical-water technologies, and have

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Table 1 Ionic constituents of Swiss swine manure solids as measured with ion chromatography after Soxhlet extraction Cation

mg/kg

Anion

mg/kg

NH4 + K+ Na+ Mg2+ Ca2+

47,000 9,100 6,700 3,800 2,700

PO4 3− NO3 − SO4 2− Cl− S2 O3 2− (COO)2 2− F− C2 H5 COO− CH3 COO−

67,000 21,000 11,000 5500 1600 1200 38 30 ND

Concentrations are given in mg/kg on a dry basis. ND: not detected.

Fig. 1. Density [6], static dielectric constant [7] and ion dissociation constant (KW ) [8] of water at 30 MPa as a function of temperature. The dielectric constant of water drops drastically as water is heated, and approaches that of a (room temperature) non-polar solvent at supercritical conditions.

been described as among the most important factors hindering the commercialization of supercritical-water oxidation [5]. 1.2. Fertilizer recovery in biomass gasification The ionic components of biomass are often rich in nitrogen, phosphorus, and potassium, which are collectively known as the N–P–K fertilizers. If these nutrient salts can be recovered in a biologically available form, they represent a potentially valuable by-product of the SCWG process. Most nitrogen fertilizer is derived from, or applied directly as, anhydrous ammonia (NH3 ) which is made by the famous Haber–Bosch process. This process reacts atmospheric N2 with H2 to form NH3 ; the H2 in turn is typically produced from natural gas (CH4 ) via steam reforming and the water-gas shift reaction. When running at stoichiometric efficiency, 3/8 of a mole of CH4 is consumed to produce each mole of ammonia, and the overall methane-to-ammonia reaction is endothermic (Hrxn =16 kJ/mol NH3 at standard conditions, not counting the heat of vaporization of water), which generally requires the consumption of more fossil fuels for heating needs. A recent study [9] found that on average, in the US, 51 MJ of energy is required per kilogram of NH3 produced. The recovery of ammonium from biomass offers an opportunity to displace fossil fuel use even further by avoiding use of the Haber–Bosch process. Table 1 shows the measured concentrations of ionic species of a manure sample considered for gasification, which shows the possible recoverable nutrient salts from biomass. Analysis is by ion chromatography after Soxhlet extraction with water. Note the large quantities of the biologically available [10] ions of nitrogen (NH4 + , NO3 − ), phosphorus (PO4 3− ) and potassium (K+ ). 1.3. Existing salt separation techniques Marrone et al. [11] have recently reviewed several technologies for the separation of salts from supercritical water. One of

these technologies, developed by Hong et al. [12] and described by Barner et al. [13], is a reverse-flow vessel, in which a brine solution enters the top of the vessel at temperatures below the critical temperature of water and is heated in the vessel. The flow reverses and exits at the top. As the flow is heated, salts precipitate and collect in the bottom of the vessel. However, these vessels can be difficult to operate, and little is known about the behavior of the solids separation inside the vessel due to the thick vessel walls necessary to contain the high pressure at these elevated temperatures. Previously, researchers have attempted to model the transport behavior inside similar vessels using computational fluid dynamics [13,14]; however, the rapid changes in fluid properties near the critical point make modeling difficult and necessitate in situ studies. Neutron transmission radiography is a technique that can be used to image this process in real time. Neutrons are capable of penetrating certain metals, including zirconium, that can be used to construct a strong vessel. In this study, we used neutron radiography to analyze, for the first time, the transport behavior of a supercritical-water salt separator. 2. Materials and methods 2.1. Imaging with neutron radiography Neutron radiographs were acquired at the thermal neutron radiography facility NEUTRA (http://neutra.web.psi.ch/) of the Paul Scherrer Institut, Villigen, Switzerland [15]. The energy spectrum of the neutrons at NEUTRA is a polychromatic, Maxwellian energy spectrum with most probable energy at about 25 meV as shown in Fig. 2. The neutron flux at the sample position was 4.95 × 106 n/cm2 /s. The collimation ratio [16] L/D describing beam divergence was 550, inducing a geometrical unsharpness of about 120 ␮m. Radiographs were recorded with a Peltier-cooled (−45 ◦ C) slow-scan CCD camera featuring a 1024 × 1024 pixel chip (camera type DV 434, Andor Technology, http://www.andor-tech.com). Neutron flux was converted into light with a Li-6 doped ZnS screen with thickness 250 ␮m. The field of view captured by the 50 mm Pentax F 1.2 normal lens was 90 mm × 90 mm, representing a nominal pixel size of 88 ␮m × 88 ␮m at full resolution and 176 ␮m × 176 ␮m after binning 2 × 2 pixels. The effective spatial resolution [17],

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Fig. 2. Linear attenuation coefficients of light and heavy water at 30 MPa and 300 ◦ C. Values for salts given for comparison at room temperature and nominal solid density. The effective spectrum averaged Σ values for an 8-mm thick water layer are about 0.38 cm−1 for heavy water and 2.76 cm−1 for light water. Note that Σi = σi ρi NA /Mi .

however, was limited by the scintillator thickness to about 400 ␮m/pixel. The exposure time was 15 s per picture providing a dynamic range of about 25,000 gray levels. This exposure time allowed for the visualization of salt accumulation, which was generally on the order of minutes. 2.1.1. Image analysis Each 15-s neutron radiography exposure produced a 512 × 512-element matrix of intensity data proportional to neutron transmission at each point on the grid. These values were converted to relative transmission by dividing each element in the matrix by the “flatfield” transmission, or transmission when no sample was present in the field of neutrons. This gave values described by the Beer–Lambert exponential attenuation law,  #materials  →  ρi I( r ) → NA σi xi ( r ) . → = exp − Mi I0 ( r ) i=1

→

I( r ) is the intensity of neutrons passing through the sample at → → point r , I0 ( r ) is the neutron intensity entering the sample at → point r , ρi , Mi , and σi are the density, molecular weight, and → neutron-attenuation coefficient of material i, xi ( r ) is the thick→ ness of material i at position r , and NA is Avogadro’s number. In this way, materials with different attenuation coefficients (σi ) can be distinguished by the intensity of neutrons passing through a sample. In particular, deposits of highly absorbing materials such as Na2 B4 O7 can be seen to accumulate in contrast to lesser absorbing materials such as D2 O. Additionally, a median filter, which replaces each intensity value with the median value of its four closest neighbors, was applied to each image to reduce noise, which can be caused by gamma rays hitting the detector. The resulting values are represented in images as shades of gray, with higher transmission taking on white shades and lower transmission taking on black shades.

In some images in this article, we have used false color1 to highlight differences between images, indicating buildup of salt deposits between two time points. In this technique, two images are used from the same run, and the neutron transmission at each pixel of the later image was divided by the transmission at the corresponding pixel in the earlier image. If the ratio of the transmittance of the second image to the first image was 0.94 or below, indicating significantly higher neutron attenuation in the second image, false color was shown at that pixel. Ratios of 0.94 were assigned a yellow color, and ratios of 0.70 and below were assigned a red color, with intermediate values given intermediate shades, as shown on the color bar of each figure. If the ratio was above 0.94, the original image was shown in grayscale. The 0.94 tolerance was found empirically to be a suitable level to show salt deposits but to not show subtle variations in fluid density or random noise from the neutron radiography method. The images resulting from the false-color manipulation show in the background a grayscale representation of the original image, with a superimposed color zone indicating areas of significant salt buildup. The color of the buildup indicates the severity of the buildup. All neutron radiography images in this article (with the exception of Fig. 3) show the width of the vessel from outer wall to outer wall, but all salt accumulation will of course only be within the inner walls of the vessel where the fluid flows. 2.2. Salt separation vessel and test rig A vessel has been constructed of nuclear-grade Zircaloy-2 (UNS R60802), which was chosen for its combination of high strength at elevated temperatures, its low neutron-attenuation coefficient and its high resistance to corrosion [18]. SITEC, of Z¨urich, Switzerland, custom fabricated the vessel to an internal diameter of 1.2 cm and internal length of 43 cm, which also included six attachment points for high-pressure fittings. A neutron transmission image of the vessel is shown in Fig. 3. Cool salt solution enters the vessel through a 0.76 / 1.59-mm internal-diameter/external-diameter dip tube extending either 2.7 or 6.3 cm into the vessel. Unless explicitly stated, the length of the dip tube was 2.7 cm for all tests reported. The vessel exit was through a side port located above the dip tube exit, 1.6 cm from the top of the vessel. This assured a reverse-flow pattern in the salt separator. A thermocouple occupied the third port in the top of the vessel to estimate the exit temperature. The bottom of the vessel was fit with a valve for use in draining and cleaning of the vessel; however, during runs, this valve was kept closed and the brine solution was allowed to accumulate. For flow conditions used in this study, we estimate the Reynolds number in the dip tube to be in the range 20–3000 (depending on preheater temperature and flow rate) and to be in the range 30–300 for the vessel as a whole, using the inner 1 Images appear in black-and-white in the printed version of the article, but are available in color on the online version.

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Fig. 3. (a) Vessel schematic and (b) neutron transmission image of the entire vessel, acquired as five separate images labeled Zone A through Zone F. This particular image shows a blockage occurring in Zone A of the vessel and dense brine accumulating at the bottom.

diameters of the dip tube and the vessel, respectively, as characteristic lengths. We estimate the Grashof number to be on the order of 1011 , indicating strong natural convection. Room temperature salt solution was fed to the system using a Waters 515 HPLC pump capable of pulseless flow in the range of 1–10 mL/min. It first entered a preheater consisting of coiled tubing surrounding an aluminum block. The preheater was capable of heating the fluid to temperatures of 350 ◦ C, providing a significant portion of the fluid heating requirements but keeping the water at subcritical temperatures. The preheater was not used in all experiments, depending on the intended operation of the vessel. The salt solution then flowed from the preheater into the vessel, where it entered through the entrance dip tube, the flow reversed, and it exited again through the top of the vessel. The vessel was heated using an aluminum block heater that surrounded the vessel in good contact with the vessel’s wall. The

aluminum block was electrically heated via cartridge heaters. The vessel and preheater temperatures were controlled with LabVIEW software (National Instruments) on a laptop computer. This remote arrangement facilitated operation of the test rig from outside of the neutron imaging chamber, which could not be entered while the neutron beam was on for safety reasons. The reduced-salt solution that exited the top of the vessel was cooled using a tank heat exchanger consisting of coiled tubing immersed in room temperature water. After exiting the tank heat exchanger, the cooled solution passed through a filter to remove any remaining suspended particles and a back-pressure regulator, which was used to keep the system at pressure. The effluent then passed through a conductivity flow cell for online monitoring of salt concentration to determine salt removal performance. Because the field of view of the neutron beam was 9.0 cm (versus a vessel height of 40 cm), the vessel needed to be moved

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Fig. 4. Simplified schematic of the experimental apparatus.

vertically within the neutron beam in order to image the whole vessel. This was accomplished by locating the vessel on a remotely movable table in front of the beam, as shown in Fig. 4. For convenience in categorizing images, the vessel was broken into five zones, labeled A–E and shown in Fig. 3. Each figure in this article is labeled with its zone which indicates its vertical position. Between runs, salt deposits were dissolved and removed from the vessel by flushing with D2 O or brine solution at temperatures low enough (below the critical point of water) to ensure high solubility of salts. The cleanliness of the vessel was verified by imaging with neutron radiography. 2.3. Chemical model system Because the most abundant natural isotope of hydrogen (1 H or H) scatters neutrons it was not considered to be a good candidate for use in neutron imaging. However, deuterium (2 H or D) has a relatively low neutron-attenuation coefficient. For this reason, heavy water, D2 O, was used as a surrogate for H2 O in neutron radiography experiments. Table 2 shows the critical parameters of light and heavy water. The ionization constant of heavy water is known to be about an order of magnitude lower than that of light water over the range of 25–250 ◦ C [19]; however, we have not been able to find data in the region near the critical point.

Three salts were used in the neutron radiography experiments: sodium tetraborate, or borax (Na2 B4 O7 ), sodium sulfate (Na2 SO4 ) and potassium phosphate (K3 PO4 ). Borax was chosen for the high contrast it would produce with the D2 O; boron has an abnormally high neutron-attenuation coefficient. Sodium sulfate and potassium phosphate were chosen as representative salts which may occur in biomass that precipitate (in their pure forms) as solids [21] and liquids [22], respectively. In order to run the separator in conditions where it may be more prone to blockage, a salt which rapidly precipitates as a solid from super-saturated supercritical-water solution was used. Sodium sulfate was chosen due to its well characterized behavior [23,21,24] as a Type-2 [25] salt with very low solubility in the range of supercritical-water conditions studied, as well as the documented problems it causes in supercritical-water gasification of biomass [4]. Table 2 Critical parameters and normal boiling point of H2 O and D2 O [20]

Critical temperature (K) Critical pressure (MPa) Critical density (g/cm3 ) Normal boiling point (K)

H2 O

D2 O

647 22.1 0.32 373.2

644 21.7 0.36 374.6

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The common salt NaCl was not used for a number of reasons: (1) chlorine is known to be quite corrosive under hydrothermal conditions and its interaction with the zircaloy vessel was unknown, (2) it has complicated phase behavior in which three phases can be in equilibrium [26], and (3) it is a less common salt occurring in feedstocks currently under consideration for biomass gasification at PSI. The anhydrous form of all salts was used to avoid introducing light hydrogen into the system.

therefore purposely operated the vessel in a regime in which it was prone to blockage, in order to study what factors influence rapid salt buildup in undesirable zones. Fig. 5 shows the behavior of the vessel when it was run with borax (Na2 B4 O7 ) alone. It can be seen that borax separates from the supercritical-water phase as a brine solution. At these run conditions, the borax brine separated easily from the supercritical water, forming a brine film that ran down the wall and collected at the bottom of the vessel.

3. Results

3.1. Operational characteristics of salt separator

Approximately 3 weeks of testing were conducted on this vessel using the neutron beam. Table 3 summarizes key conductivity observations in these runs. During this time, attempts were made to understand the effects of the various operating parameters on the performance of the vessel. In particular, it was desirable to understand the causes and conditions that can lead to blockage of the vessel, which generally resulted from accumulation in Zones A, B, and C of the vessel. (In normal operation, accumulation should occur in Zones D and E.) We

The separator has also been run with light water, H2 O, in order to characterize its performance. The vessel was fit with a 1.52-mm (outer diameter) thermowell running axially along the centerline of the vessel (although due to the thin nature of the thermowell, some deviation from the centerline was observed through subsequent neutron radiography images). Fig. 6 shows the thermowell temperature recorded versus position in the separator for three flow-rates running with salt-free H2 O. As expected, the upper zone temperature is observed to decrease

Table 3 Run conditions and conductivity reductions Vessel wall temperature (◦ C)

Preheater temperature (◦ C)

Pressure (MPa)

Flow rate (mL/min)

Dip tube length (cm)

Conductivity reduction

Flow rate effects A 1% Na2 SO4 , 1% Na2 B4 O7 B 1% Na2 SO4 , 1% Na2 B4 O7 C 1% Na2 SO4 , 1% Na2 B4 O7 D 1% Na2 SO4 , 1% Na2 B4 O7 E 1% Na2 SO4 , 1% Na2 B4 O7 F 1% Na2 SO4 , 1% Na2 B4 O7

450 450 450 450 450 450

Off Off Off Off Off Off

30 30 30 30 30 30

1 2 3 3 5 10

2.7 2.7 2.7 6.3 2.7 2.7

Plug Plug 83% 85% 63% 26%

Preheater effects G 1% Na2 SO4 , 1% Na2 B4 O7 H 1% Na2 SO4 , 1% Na2 B4 O7 I 1% Na2 SO4 , 1% Na2 B4 O7 J 1% Na2 SO4 , 1% Na2 B4 O7 K 1% Na2 SO4 , 1% Na2 B4 O7 L 1% Na2 SO4 , 1% Na2 B4 O7 M 1% Na2 SO4 , 1% Na2 B4 O7

450 450 450 450 450 450 450

Off 330 Off 300 300 300 330

30 30 30 30 30 30 30

10 10 10 10 5 3 3

6.3 6.3 2.7 2.7 2.7 2.7 6.3

48% 73% 26% 69% Plug Plug Plug

Vessel temperature effects N 1% Na2 SO4 , 1% Na2 B4 O7 O 1% Na2 SO4 , 1% Na2 B4 O7

450 475

Off Off

30 30

5 5

2.7 2.7

63% 81%

Pressure effects P 1% Na2 B4 O7 Q 1% Na2 B4 O7 R 1% Na2 B4 O7

450 450 450

Off Off Off

30 25 20

1 1 1

2.7 2.7 2.7

85% 89% 97%

Potassium phosphate effects S 1% K3 PO4 , 1% Na2 B4 O7 T 1% K3 PO4 , 1% Na2 B4 O7

450 450

Off 300

30 30

3 3

2.7 2.7

71% 82%

Light water effects U Pure H2 O V Pure H2 O W Pure H2 O

450 450 450

Off Off Off

30 30 30

1 5 10

2.7 2.7 2.7

N/A N/A N/A

Higher concentrations effects X 3% Na2 SO4 , 1% Na2 B4 O7 Y 3% Na2 SO4 , 1% Na2 B4 O7

450 450

Off 300

30 30

3 3

2.7 2.7

Plug Plug

Run ID

Feed

All runs used D2 O unless specified. All vessel temperatures given are upper-zone wall set-point temperatures. Percents are weight percents.

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1 g/min flowrate carry more thermal energy into the lower zones (>22 cm from the top) of the vessel, resulting in higher temperatures in the lower zones. However, the 5 g/min condition results in a significantly higher temperature in the upper zones than the 10 g/min condition, so the fluid being carried into the lower zones is hottest at 5 g/min. 3.2. Effects of salt type Three different salts were used in experiments involving neutron radiography, Na2 B4 O7 , Na2 SO4 , and K3 PO4 . As mentioned above, Na2 B4 O7 was required in order to have high-contrast images of the salt separation, and was used in most of the neutron radiography tests. When fed alone, Na2 B4 O7 was observed to separate as a liquid condensate, as shown in Fig. 5. The salts K3 PO4 and Na2 SO4 were used as representative salts that form liquid (or dense fluid) and solid precipitates, respectively. The vessel was found to operate without significant problems in tests with K3 PO4 , similar to the operation with Na2 B4 O7 alone; thus, most testing focused on the use of Na2 SO4 with Na2 B4 O7 used as a tracer. This system led to many solid precipitation issues and should be representative of a worst case scenario for the salts that are present in real biomass, such as manure. 3.3. Precipitation and condensation Fig. 5. Borax separation from supercritical water. The borax formed a film that ran down the vessel walls. Run condition P (see Table 3); Zone B pictured (see Fig. 3).

with increasing flow rate as the thermal load becomes greater. However, the lower zone temperature shows some inversion, with the 10 g/min profile coming between the 5 and the 1 g/min profiles. A possible explanation follows: as expected, the higher momentum of the 5 and 10 g/min flowrates as compared to the

An interesting observation of this study is that salt condensation/precipitation does not occur exclusively at localized hot surfaces within the separator. At localized hot zones the solubility of salts will be at the lowest of any point in the vessel, which, as expected, can lead to precipitation or condensation, often of a solid phase. However, many salts can also form a liquid brine phase with relatively high salt concentrations that is stable at lower temperatures. Therefore, a brine phase can form at localized cool zones within the vessel. Fig. 7 (a) shows precipitation occurring at the thermocouple, which is located 2 cm above the top of the heater and presumably would not be a localized hot zone; in fact, it may be one of the cooler zones in the upper part of the vessel. Additionally, a drop is visible at the exit of the dip tube; this is likely just subcooled liquid that is still cold due to the lower temperature of the entrance conditions. This system was the binary system D2 O/Na2 B4 O7 . Fig. 7(b) shows precipitation on the walls of the vessel for the ternary system D2 O/Na2 B4 O7 /Na2 SO4 . Since the contents of the vessel are heated at the walls, this accumulation is assumed to be precipitation due to the lower solubility of salts at higher temperatures. 3.4. Effect of flow rate

Fig. 6. Thermowell temperature versus vertical position in the vessel for salt-free light water (1 H2 O) runs (run conditions U, V, and W).

The flow rate has a large influence on the performance of the separator. Fig. 8 shows the accumulation of Na2 SO4 /Na2 B4 O7 deposits in the vessel for a range of flow rates. The deposit moves further into the vessel at higher flow rates, indicating the flow penetrated more deeply into the vessel at higher momentum. The higher momentum is caused not only by the faster velocity,

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Fig. 7. Examples of the type of precipitation and condensation that occur in the supercritical-water salt separator. (a) Shows condensation of a cooler liquid brine phase at the cooler points in the separator, the thermocouple and the entrance dip tube (run condition P). (b) Shows precipitation of solid salts on the hot walls of the vessel due to their lower solubility caused by the locally higher wall temperatures (run condition C).

but also by the higher density of the high flow-rate stream, since it cannot be heated as rapidly after entering the vessel. In the extreme case of 10 mL/min, very little accumulation of solid salt deposits occurred; this is likely due to the lower temperature of the fluid inside of the separator which prevented the drop in solubility associated with higher temperatures. This is consistent with the temperature profiles observed at different flow rates in Fig. 6. 3.5. Length of entrance dip tube The effect of the length of the entrance dip tube was tested experimentally. Two sizes of dip tube were used: 2.7 and 6.3 cm in length, as measured from the top of the vessel. At the lower flow rates tested, the shorter dip tube gave equivalent or better performance; only at the maximum flow rate of 10 mL/min did the long dip tube perform better. At relatively low flow rates, it is likely that the fluid in the dip tube was heated by the vessel contents, leading to salt precipitation, which was observed with neutron radiography to occur very near the dip tube exit.2 In future tests, it may be possible to circumvent this issue by using an insulated entrance dip tube. In principle, the length of the dip tube should be a balance: too short of a dip tube would cause the flow to “short-circuit” and exit without penetrating far into the vessel, and too long of a dip tube could lead to precipitation of salts within the dip tube as the brine solution heats up.

Fig. 8. The effect of flow rate on the location of buildup in the vessel. With other operating conditions constant, the flow rate was set at (a) 1, (b) 3, (c) 5, and (d) 10 mL/min. Note that the buildup in images (a–c) occurred in Zone A, but the buildup in image (d) occurred lower, in Zone B (run conditions A, C, E, and F, respectively).

3.6. Blockage causing a shorter effective residence time The complete vessel shown in Fig. 3 shows a case where a blockage formed part way down the vessel, which likely resulted in a shorter average residence time of the fluid in the vessel. The reduced residence time prevented the fluid from being heated to the supercritical temperatures at which the bulk of the salt separation occurs. In this run, a sudden onset of a rise in exit conductivity and a decline in exit temperature were observed corresponding to the buildup of this blockage.3 The conductivity and exit temperature changes are shown in Fig. 9.

2

A video of accumulation showing the buildup of salt at the base of the dip tube, and above it, is in the supplemental online materials, under filename RunM.avi.

3 A video showing the blockage and subsequent brine accumulation is in the supplemental online materials, under filename RunX.avi.

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References

Fig. 9. Observed conductivity and exit temperature from run condition X.

Below the blockage, the vessel was relatively stagnant, resulting in a low density fluid with some borax accumulation on the walls. 4. Conclusion Neutron radiography can effectively be used as an imaging tool to help understand complex precipitation phenomena in supercritical water. Salts that condense as liquid brines (Na2 B4 O7 , K3 PO4 ) were observed to separate in the salt separator as desired. Na2 SO4 , a Type-2 [25] salt that precipitates as a solid from supercritical water, was observed (with the aid of a Na2 B4 O7 tracer) to often cause blockage of the vessel. Neutron radiography was successfully used to identify the onset and location of salt precipitation that may lead to blockage. As a result, this method could be used to establish a range of suitable operating conditions for the supercritical-water salt separation vessel. Acknowledgements We would like to thank Erich DeBoni and Peter Hottinger for their work in building the supercritical-water apparatus, Dr Eberhard Lehmann for his assistance with the neutron radiography, and Maurice Waldner and Thanh-Binh Truong for general assistance. We would like to acknowledge funding from the Martin Family Society of Fellows for Sustainability, and the Society for Energy and Environmental Research through U.S. Department of Energy award DE-FG36-04GO14268. Support by the DOE does not constitute endorsement by the DOE of the views expressed in this article. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online versions, at doi:10.1016/j.supflu.2007.08.003.

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